#app {
    width: 100%;
    height: 100%;
    background-color: red;
}
.allbox{
    width: 1200px;
    height: 200px;
    margin: 0 auto;
    background-color: aliceblue;
    display: flex;
    justify-content: space-between;
}
.left,.right{
    width: 300px;
    height: 100%;
    background-color: blue;
    line-height: 200px;
    text-align: center;
}
.center{
    width: 100%;
    height: 100%;
    background-color: pink;
    line-height: 200px;
    text-align: center;
    flex: 1;
}
.allbox2{
    display: flex;
    width: 100px;
    height: 200px;
    flex-direction: column-reverse;
}